Key Design Parameters And Engineering Application For Helical Piles

In the context of global green and low-carbon,concrete piles commonly used in railway subgrades are being challenged.Therefore,there is an urgent need for a pile foundation that is environmentally friendly,easy to construct and has high bearing efficiency as an alternative.In recent years,the continuous development of helical piles may become an alternative.The helical pile is composed of a central shaft（i.e.,pipe or solid rod）and a number of helices welded on it.Helical piles are installed by rotating and pressing in,so the construction is fast and does not produce spoil.It can be loaded immediately after installation.Helical piles have potential application value for the existing railways or new lines.Although there are many reports on the theory of axial bearing design of helical piles,the research on its key design parameters（e.g.,inter-helix spacing,settlement,installation power）is not precise and reasonable,and helical piles are rarely used in railway reinforcement.Funded by the National Natural Science Foundation of China（52078427 and 51978588）and the Science and Technology Innovation Project of Shenhua Baoshen Railway Group Corporation（SHGF-17-46）,this thesis carried out the following research work on the critical design parameters and engineering applications for helical piles.1.optimal inter-helix spacing（Chapter 2）Based on the Meyerhof pile foundation theory and the cavity expansion method,the influence zone of the helices in sand is analyzed to determine the optimal inter-helix spacing.The Meyerhof pile foundation theory considers the strength parameters of sand and uses a logarithmic spiral to define the failure zone near the helix.The cavity expansion method considers the fine content,soil compressibility and stress level,combined with small strain stiffness and uses the plastic limit zone of the cavity to determine the influence zone near the helix.The results show that the optimal inter-helix spacing determined by Meyerhof pile foundation theory is quite different from the spacing range in the existing literature.In the cavity expansion theory,clean silica and silt sand are used as examples to illustrate that the influence zone of the helices in sand is consistent with the literature analysis results.The optimal spacing determined based on the cavity expansion method is verified in the centrifuge test of the helical piles in Congleton HST95 sand.A finite element model based on hardened soil with small strain stiffness and the helical pile is established.The finite element calculation results are consistent with the optimal spacing based on the cavity expansion method.2.Evaluation criteria for ultimate bearing capacity and finite element calculation model considering installation effect（Chapter 3）Based on the load test of the helical pile,the differences in the domestic and foreign methods of determining the ultimate bearing capacity are discussed.Secondly,according to the cone penetration test results,the soil parameters are evaluated,and the numerical calculation model considering the installation effect for the load test is established.The calculated load-settlement curve is compared with the measurement.Finally,based on the numerical calculation results,the variation law of axial force and side friction resistance along with the pile depth under different load levels are analyzed.Likewise,the size and proportion of the side friction and end resistance are analyzed.The results show that the domestic method is more suitable than the two foreign methods in evaluating the ultimate bearing capacity for helical piles using domestic pile foundation testing procedures.The finite element calculation result considering the installation effect is closer to the measurement than the result without considering the installation effect.Under vertical load,the side friction resistance of the central shaft and the end resistance of the upper and middle helices play a role simultaneously.As the load increases,the end resistance of the lower helix and the bottom of the central shaft continues to play,and the sharing ratio gradually increases.3.load-settlement behaviour（Chapter 4）Based on the nonlinear degradation of soil stiffness with shear strain,the load-settlement nonlinear behaviour of helical piles in sand is evaluated.The Randolph analytical pile solution is mainly used as the theoretical framework.Based on the load-settlement curves of 9 single-helix piles in sand with different densities,the back-analysis method is used to calculate the soil stiffness corresponding to different pile settlements.At the same time,based on the parameters of the test site in-situ test（i.e.,CPT or SPT）,the linear relationship between the initial shear modulus of the soil layer within the pile depth is determined.After that,the soil stiffness and the corresponding pile settlement are normalized,and the nonlinear relationship between the soil stiffness and the settlement for the helical pile is obtained,and the relationship is compared with the existing driven pile.Finally,the empirical relationship for stiffness varying nonlinearly with the settlement is used to predict the load-settlement behaviour of the helical pile in 9 centrifuge model tests and four field tests.The results show that the prediction results of the proposed empirical relationship are in good agreement with the measured results in the centrifuge test and the field test.4.installation power of helical pile（Chapter 5）A new method for the installation torque and force prediction of single-helix and multi-helix piles is proposed.The method considers four factors:the relationship between the vertical and the tangential shear stress on the side of the central shaft during rotary press-fitting,helix installation effect,the interaction between the helices and the vertical installation force affect the installation torque.The installation torque and power for helical piles in sand with different relative densities（loose,medium,and dense）are predicted using the proposed method and the two existing methods（Ghaly and Hanna method and Al Baghdadi method）.The prediction of the three methods is compared with the results of centrifugal tests of screw piles.The prediction by using the proposed method is in good agreement with the measured data.At the same time,the prediction by using the proposed method are in good agreement with the measurement of the on-site installation for the double-helix pile.In addition,the law of the installation power along with the depth for the helical pile with the sand relative density and the number of helices is analyzed,and the contribution of the installation force and the installation torque of the central shaft and the helices to the installation power is analyzed.5.Effect evaluation and settlement estimation method for railway subgrade is reinforced with oblique grouted helical piles（Chapter 6）The design ideas and construction technology of oblique grouted helical piles for strengthening existing railway subgrades are introduced.The backfill soil before subgrade reinforcement is tested in situ（i.e.,CPT）to evaluate its physical and mechanical properties.The foundation coefficient K₃₀ was tested at three measuring points before and after the subgrade reinforcement.A static level was used to monitor the settlement of 5 measuring points in the reinforced section of the roadbed.The foundation coefficient K₃₀ of 3 measuring points after screw piles strengthened the roadbed increased by 48%to 166%.The long-term settlement of the railway embankment after reinforcement shows that after an adjustment period of about half a year,the settlement is gradually stabilized,and the settlement at the center of the reinforcement section of the embankment is the largest,with a maximum settlement of 2.76 mm.Based on the improved CPT method to predict the settlement of shallow foundations,combined with a simplified calculation model of inclined piles to strengthen the subgrade,the settlement of the railway subgrade is estimated.The predicted settlement is 3.725 mm,which is in good agreement with the measured value of 2.76 mm.6.the dynamic response for helical piles obliquely strengthening railway subgrade（Chapter 7）Accelerometers and speedometers are used to test the dynamic response at the edge and middle of the sleeper.For a C_80B wagon with a speed of 72 km/h and an axle load of 25 t,the vibration parameters of the subgrade were tested before and after the existing subgrade was strengthened.Time-domain and frequency-domain analyses of vibration parameters are performed,and 1/3 octave spectrum is introduced to evaluate the vibration level.Time-domain analysis shows that the variation range,average value and standard deviation of the root mean square value of acceleration,velocity and displacement after reinforcement is all reduced.Frequency domain analysis shows that the distribution law of the characteristic frequency before and after strengthening is the same,but the corresponding amplitude of characteristic frequency becomes smaller after strengthening.The frequency bands with obvious attenuation of vibration level are the centre frequency of 1.6 Hz and 50 Hz,respectively.